key1

; Assemble with:
;     nasm -f bin key.asm -o key

start:
    mov bx, [0x0DED]         ; Get the WORD from [0x0DED]
    neg bx                   ; Negate it
    nop                      ; Need to wait at least one more instruction (3 total)
                             ;     before trying to read 0x0DED again
    add bx, [0x0DED]         ; Add the current WORD value at [0x0DED] with BX
                             ;     safeAX = -BX+[0x0DED] = [0x0DED]-BX
                             ;  or safeAX = (-prevsafeAX)+[currentsafeAX] = [currentsafeAX]-(prevsafeAX)
    mov byte [bx], 060h      ; Overwrite the first instruction of safe with 060h
                             ;     to terminate the safe.
    jmp $                    ; Infinite loop